The exterior doors for homes, apartments, condos and the like are usually made of wood, such as a hardwood, and may have a metal cladding to form a composite laminated structure. In lieu of a wooden substrate or core, a suitable foam plastic may be employed. While generally satisfactory for the purposes intended, these doors have relatively poor impact resistance, hence are subject to vandalism and burglary. A single hammer blow, or at most a short series of hammer blows, is usually all that is required to smash an exterior door. The average wooden door or residential steel door can usually be kicked open by a single blow of the foot.
In some installations, the doors have to be replaced repeatedly. For example, the City of Baltimore presently replaces about 3000 entrance doors per year in apartment projects alone. Extrapolating this experience on a national level indicates that several hundred thousand entrance doors must be replaced on an annual basis. While age alone accounts for a small percentage of this total, most of these doors are replaced because of vandalism or forced entry. Thus door replacement is both aggravating and expensive, and a solution to this vexing problem has eluded the prior researchers.
In an effort to control such wanton vandalism, I have experimented for over ten years. Literally hundreds of materials and construction modes have been tested in an effort to develop an economically feasible door that could withstand repeated abuse. I have also experimented with the use of relatively high-impact resistant materials for the outer surface or face of a door, thereby forming a laminated structure. One such material is a polycarbonate manufactured and sold by General Electric Company under its trademark "LEXAN". This material has been used in other applications, such as laminated structures for high-impact resistant (or "bulletproof") glass.
However, problems have been encountered in suitably adapting a polycarbonate sheet for a laminated door. First, if the polycarbonate sheet is bonded to only the exterior surface of the door, the door will tend to warp, even though the polycarbonate sheet itself will not crack or shatter. Second, the polycarbonate sheet and the substrate (or core) of the door will have different coefficients of expansion. Thus, with the temperature variations experienced from the winter to summer seasons, the polycarbonate sheet will tend to warp or separate from the door substrate; and the adhesives available in the marketplace have been totally unsatisfactory in maintaining a good bond between the polycarbonate sheet and the door substrate.